The field of the invention relates to the manufacture of display devices. More specifically, the present invention pertains to producing a lacquer layer in the manufacture of display devices.
For over 30 years, companies have searched for ways to construct a thin, low-power version of the conventional cathode ray tube (CRT). These efforts have led to a number of flat panel display technologies. None, including liquid crystal displays (LCDs) have met all of the needs for improved power, brightness, efficiency, video response, viewing angle, operating temperature, packaging, full color gamut, ruggedness, and scaleability.
Among the obstacles encountered in fabricating thin cathode ray displays is the deposition of a lacquer layer on the faceplate of the display prior to adding an aluminum layer. The aluminum layer is used to act as a mirror behind each sub-pixel in the display faceplate to reflect the light photons back toward the viewer of the display screen to create a brighter image. Surface irregularities in the aluminum layer scatter these photons and reduce the efficiency of the aluminum layer in reflecting light to the viewer, thus degrading the brightness of the display. The lacquer layer provides a supporting structure when the aluminum layer is deposited so that the aluminum layer is deposited upon an even surface and will reflect light evenly toward the viewer.
One method of depositing the lacquer layer is known as a xe2x80x9cfloat lacquerxe2x80x9d process. FIGS. 1A-C are cross section views showing the steps in a prior art float lacquer process. In FIG. 1A, a faceplate 101 is submerged in a solvent 102 such as water. In FIG. 1B, a thin layer of lacquer 103 is deposited or floated on top of water layer 102. The water is then drained from the tank and, as the water level subsides, lacquer layer 103 is deposited upon faceplate 101. In FIG. 1C, the level of water layer 102 in the sub-pixels 104 of faceplate 101 is then further reduced by evaporation and an aluminum layer is deposited directly on top of lacquer layer 103. If the aluminum layer were to be deposited directly upon the phosphor particles within sub-pixels 104, it would conform to the surface of the phosphor particles and have a very irregular surface which would reflect light back to the phosphor particles unevenly. During a subsequent baking operation, the remnants of lacquer layer 103 are removed as they can absorb electrons from the cathode and cause phosphor degradation if they remain.
The float lacquer process, however, is time consuming and is vulnerable to operator error. The amount of time it takes to set up the float tank and allow the water to become still enough to deposit lacquer layer 103 means the process is not well suited to larger scale manufacturing processes. Additionally, there can be variations in lacquer layer 103 as large as 30% using the float lacquer process, resulting in an irregular aluminum surface. This causes a non-uniform screen appearance and degrades the efficiency and brightness of the display.
The structure of thin CRTs limits the choice of lacquers in a float lacquer process to soft materials with very high elongation. High elongation is necessary to obtain a scaffold for the reflective aluminum to be applied without xe2x80x9ctentingxe2x80x9d over the sub-pixel regions. Tenting can be caused by an excessive amount of lacquer on the faceplate which makes the surface of the aluminum balloon and rupture when the lacquer and remaining water is baked out. Tenting can be detrimental, not only to the faceplate, but also during final assembly when support structures, inserted to provide greater structural integrity, can cause the aluminum layer to break which leads to electrical arcing in the finished display assembly. Tenting causes non-uniform screen appearance and reduced efficiency and brightness.
FIG. 2A shows an exemplary display screen 200 which has undergone aluminum layer deposition and solvent bake out. The surface of aluminum layer 250 overlies sub-pixel areas 203 containing phosphor particles 202. Aluminum layer 250 has undergone tenting in the sub-pixel regions during the bake out step and now has a convex surface profile from the viewers direction (the direction of arrow 260) rather than a flat surface. Due to the convex profile, light photons will now be scattered by the aluminum layer rather than directed to the viewer and the efficiency and brightness of the display are thus decreased.
Materials with high elongation are also soft materials, which means that the lacquer layer will be very conformal around the phosphor particles in the sub-pixels. In FIG. 2B, a highly conformal lacquer layer 201 has been deposited upon a layer of phosphor particles 202 contained in a sub-pixel 203. An aluminum layer deposited upon this lacquer layer will take on the shape of the conformal lacquer layer during the subsequent baking step to remove the lacquer layer and any remaining solvents. This causes the aluminum to also take on an irregular shape which reduces the reflectivity of the aluminum layer and can cause a grainy appearance in the display due to bad uniformity. To smooth the aluminum, a thicker lacquer layer ( less than 1 xcexcin thickness) is usually deposited on a regular CRT. Due the lower voltages used in a thin CRT, a thinner layer of aluminum is necessary to prevent excess electron energy loss. However, this thin aluminum layer is susceptible to blistering and breakage during the bake out if the lacquer layer is greater than 1 xcexcin thickness. In summary, using a thin lacquer layer creates an excessively conformal aluminum layer and using a thicker lacquer layer leads to tenting and rupturing of the aluminum layer.
Accordingly, the need exists for a method for depositing a lacquer layer in the sub-pixel areas of a display device which will result in a smooth, highly reflective aluminum layer that is electrically and mechanically robust. It is also desirable that this method, while meeting the above stated needs, should be applicable to large scale manufacturing processes.
The present invention is a method for selectively removing a lacquer layer so that so that the remaining lacquer is disposed in the sub-pixel areas of a display device, resulting in a smooth, highly reflective aluminum layer that is electrically and mechanically robust. It is also desirable that this method, while meeting the above stated needs, should be applicable to large scale manufacturing processes.
In one embodiment, a layer of thermally degradable, photo-imageable lacquer is deposited on top of a faceplate of a display device. Portions of the lacquer layer are etched and removed using photolithography methods and selected portions of the lacquer layer remain deposited in the sub-pixel areas of the faceplate. This remaining layer will then later be decomposed thermally and degraded into volatile products that will disappear during subsequent vacuum processes.
In another embodiment, the faceplate of the display device is used as the mask for defining which portions of the photo-imageable lacquer layer remain in the sub-pixel areas. This has an added advantage in that a mask does not have to be created and aligned over the faceplate to image the lacquer layer.